What does Paul mean in 1 Corinthians 14:15 when he says I will pray with the spirit and I will pray with understanding?
Paul means he will pray from his own innermost spirit and also pray with understanding, not in an unknown tongue that leaves the mind unfruitful. Praying in a language one does not understand cannot edify the understanding, so prayer should be intelligible to the one praying.
Les explains that Paul is speaking in the first person in 1 Corinthians 14. In verse 14, Paul says that if he prays in an unknown tongue, his spirit prays, but his understanding is unfruitful. Then in verse 15 Paul says, “I will pray with the Spirit,” meaning he will pray from his innermost being—his own small spirit—and he will also pray with understanding. Les asks what good it does to talk to God in something you do not know what you are talking about, even if God may be able to discern it, if you cannot. The point is that prayer should not bypass the understanding; the one praying should know what he is saying.
What is it then? I will pray with the spirit, and I will pray with the understanding also: I will sing with the spirit, and I will sing with the understanding also.